The immune system consists of two general features. These are: 1) the innate immune system and 2) the adaptive (antibody-mediated) immune system. The innate system represents the first line of defense against an invading pathogen (whether bacterial or fungal) and provides the adaptive immune system with enough time (3-5 days) for it to build up antibodies which are used to “fight” pathogens. While the innate and adaptive systems are often described separately, they function in tandem; striving to sequester and neutralize a pathogen challenge.
The innate immune system. The innate immune system consists of several interesting components: Aspects include:
1. Physical and chemical barriers to pathogens provided by epithelium, gastric acid and digestive enzymes.
2. Cells which engulf and digest invading pathogens (e.g., neutrophils).
3. Receptors on the surface of these cells which recognize and bind to pathogens.
4. Signaling molecules (e.g., chemokines, cytokines) which communicate sites of infection and regulate expression of immune genes.
Neutrophils. Neutrophils are among the most important cells of the innate immune system. They are the first cell to arrive at a site of infection. In mammals, there are billions of neutrophils of which about one-half are freely circulating in the blood (Burton and Erskine, 2003). The remainder are held in reserve in bone marrow where they are formed. Neutrophils express an extracellular binding protein on their membranes termed “L-selectin” (also termed CD62L). The role of L-selectin is to interact weakly with the endothelial cell wall thereby allowing the neutrophil to “roll” along the wall of a blood vessel and to “monitor” the cell wall for the presence of signals which indicate a local infection (FIG. 1). The presence of pathogens in peripheral tissues causes release of local chemicals which then signal a rolling neutrophil of an infection. In response to these signals, L-selectin is shed from the surface of the neutrophil (see FIG. 1) and other more adhesive molecules are expressed on its surface. These molecules essentially “glue” the neutrophil within the blood vessel adjacent to the site of infection. The activated neutrophil then migrates through the endothelial cell wall toward the invading pathogen. Interleukin-1β is produced by the neutrophil as a pro-inflammatory cytokine. This aids in mediating inflammation and in facilitating containment of invading pathogens. During neutrophil migration, chemical signals originating from the site of infection (such as TNF-β and interferon γ) activate the neutrophil to become a mature “killer cell”. The mature neutrophil migrates toward the site of infection where it interacts with pathogen-associated microbial patterns (PAMPs) on the surface of pathogens via several types of receptors. These receptors are expressed on the surface of the neutrophil and include the following well-identified types (FIG. 2):
a—CD18 and CD14
b—Toll-like receptors (TLRs)
c—C3b and C3bi (complement factors)
d—Fc
Binding of neutrophils to pathogens via receptors. Both CD14 and CD18 receptors bind with lipopolysaccharide (LPS), a common polysaccharide structure associated with membranes of gram-negative bacteria. In addition, neutrophils express toll-like receptors (TLRs) which recognize and bind to additional structures associated with pathogens. So far, ten different toll-like receptors have been identified in mammals (FIG. 2 and Table 1). TLRs play a critical role in early innate immunity to invading pathogens by sensing microorganisms. These evolutionarily conserved receptors recognize highly conserved structural motifs only expressed by microbial pathogens, called pathogen-associated microbial patterns (PAMPs: Invivogen, 2004). Stimulation of TLRs by PAMPs initiates a signaling cascade that involves a number of proteins, such as MyD88 and IRAK (FIG. 2). This signaling cascade leads to the activation of the transcription factor NF-kB which induces the secretion of cytokines that direct the adaptive (i.e., antibody-mediated) immune response. TLRs are predominantly expressed in tissues involved in immune function, such as spleen and peripheral blood leukocytes, as well as those exposed to the external environment such as lung and the gastrointestinal tract. Ten human and nine mouse TLRs have been characterized, seven of which have had their ligands identified. For example, TLR2 is essential for the recognition of a variety of PAMPs, including bacterial lipoproteins, peptidoglycan, and lipotechoic acids. TLR3 is implicated in virus-derived double-stranded RNA. TLR4 is predominantly activated by lipopolysaccharide. TLR5 detects bacterial flagellin and TLR9 is required for response to unmethylated CpG DNA (Table 1). Recently, TLR7 and TLR8 were shown to recognize synthetic antiviral molecules. These receptors are essential elements in host defense against pathogens by activating the innate immunity (Invivogen, 2004).
Bovine TLRs. Relatively few studies on PAMPs have been completed with bovine cells. So far, bovine immune cells have been reported to contain TLR2 and TLR4 (Werling et al., 2004). Polymorphisms have been reported in bovine TLR4 which may determine susceptibility to bovine respiratory disease and Johne's disease (White et al., 2003).
C3b and C3bi are components of the complement cascade whereas the Fc receptor binds to the “constant region” of antibodies. Hence, pathogens which are coated with complement factors or antibody (i.e., pathogens which are opsonized) are also recognized by activated neutrophils and are subsequently phagocytosed. In other words, activated neutrophils possess several means by which they recognize pathogens (Table 1).
Phagocytosis and killing. The binding of neutrophils, (and other phagocytic cells) to cell-surface markers of pathogens via these receptors, then permits the phagocytic cell to engulf the invading pathogen and “kill” it (FIG. 3). Presently, two mechanisms for “killing” are known. These include: 1) an oxidative burst, where the phagocyte expresses reactive oxygen species which destroy the phagocytosed pathogen, and 2) fusion of the engulfed pathogen with a lysosome-like structure to form a “phagosome”. The phagosome is rich in digestive enzymes which mediate complete digestion of pathogens.
Common infections. Mammalian and avian species are continually challenged by pathogens in the gastrointestinal track and in the lung. These are important sites for resident neutrophils where minimize pathogen invasion. In addition, the mammary gland of mammals represents a site for pathogen challenge. In all infections, the innate immune system plays a key initial role in fighting-off the initial immune challenge. The innate system is essential to allow the adaptive (antibody-mediated) system to develop and mount a more-specific and directed immune response.
Cooperation between the innate and acquired immune system in ruminants. Antibodies which are specific to an invading pathogen leak into a site of infection to optimize clearance of a pathogen. Individuals with a high titer against a specific antigen are able to deliver these antibodies into the site of infection via a leaky endothelium (arising from an inflammatory response). Arrival of reactive antibodies (i.e., IgG2) in the alveolus coats (opsonizes) the pathogen and, as noted previously, allows neutrophil recognition of pathogens via Fc receptors (Table 1) and phagocytosis.
Stress and immune function. Stress reduces individuals' abilities to fight disease. The negative effects of stress on the immune system are mediate by the steroidal stress hormones (Cortisol, hydrocortisone and corticosterone). Burton and co-workers at Michigan State University (Weber et al., 2001) have identified the mechanism by which stress brings about a reduction in immune function. Specifically, they have documented that glucocorticoids (i.e., cortisol) “spike” near parturition (FIG. 4) and reduce L-selectin expression in neutrophils (FIG. 5). This compromises one important aspect of an individual's first line-of-defense against pathogen challenge. Specifically, a stressed, immunosuppressed individual has reduced ability to monitor endothelial cell lining for sites of infection and to attack and to sequester pathogens. This may result in an infection (FIG. 6).